The objective of this study was to investigate the role of organic cation transporters (OCTs) in the uptake of hydrophilic drugs into the olfactory bulb and subsequently to the brain. Two OCT2 substrates, amantadine and cimetidine were used as model drugs for this purpose.
Bovine nasal explants (olfactory and respiratory tissue) were used as an in vitro model for preliminary screening to identify the role of transporters involved in the uptake of drug across these tissues. It was observed from both PCR and immunohistochemistry that OCTs, OCT2, OCTN1 and OCTN2 were present in the bovine respiratory and olfactory mucosa. Transport studies of amantadine in the presence and absence of OCT2 and OCTN2 inhibitors indicated that both these transporters play a role in the transport of amantadine across the bovine respiratory mucosa, whereas transport across the olfactory mucosa was predominantly via OCT2.
This was followed by in vivo studies in rats where the blood, striatum and olfactory bulb concentrations of amantadine were determined following intranasal and intra-arterial administration. Shortly after nasal administration, the olfactory bulb concentrations exceeded the concentrations in the striatum suggesting the olfactory pathway to be the major route of uptake. Co-administration of the drug with an OCT2 inhibitor intranasally showed statistically significant reductions in the brain uptake of amantadine. A synergistic inhibitory effect on amantadine uptake was observed with the combined inhibition OCT2 and OCTN2. Additionally, the CNS exposure of these drugs following intranasal administration in the presence and absence of the OCT inhibitors was evaluated using the ratio of the free drug concentrations in the brain compared to plasma. While the plasma concentration profiles were similar both in the presence and absence of inhibition, the free drug ratios were highest when no inhibitor was included. Additionally similiar in vivo studies were also carried out for a second model drug, cimetidine, where cimetidine uptake into the rat brain was found to be significantly reduced in the presence of the OCT2 inhibitor, pentamidine.
This demonstrates that there was a greater CNS exposure to each drug when OCT transporters were active, confirming their role in their direct CNS distribution from the nasal cavity to the brain. The results of this study suggest that OCT substrates might be good candidates for the delivery to the brain via the olfactory route.